kvserver: sep-raft-log: revisit throughput under separate raft log

[tbg]

Sumeer ran a toy experiment on raft log separation in master...sumeerbhola:cockroach:sep_lsm that also simulated loosely and tightly coupled truncations. The results didn't make sense. We should revisit them.

See #16624 (comment).

via @sumeerbhola:

btw, the thing to investigate that I was mentioning in camp roach is the sync latency mentioned in section 3 here #16624 (comment) and for which there are later comments by Nathan and Peter. We should see higher throughput with the 2 engine scheme, which we are not. We need to get to the bottom of that mystery.

Epic: CRDB-220

Jira issue: CRDB-24806

[blathers-crl]

cc @cockroachdb/replication